actually, i think it sort of depends on what you are doing. if you are just growing chaeto, you just need enough flow to tumble the clump of it. if you do one of these more elaborate "display" fuges (or run a deep sand bed or something) you would probably want more flow so it doesn't just trap detritus.
easiest way to do this is to tee off your return line, or to split a line off your drain (though that tends to be noisy/cause trouble with the durso in my experience)